Two time machines in the beginning?

I'm trying to decipher the sequence of events seen in the old video and the end of the movie. There's something not right.

What we can see in the video:
1. Party goes on
2. David appears in the mirror
3. David's dad (Ben from now on) gets a phone call, goes to the door with case in hand
4. Ben gives young David his watch and leaves.

In the end we learn that:
After 2., David goes down to the basement. Ben is already there, even though he was upstairs at the party only seconds before. He's opening or closing the floor locker. They talk, then David proceeds to destroy the machine and the documentation, Ben goes upstairs.

There's a stealth cut between 3. and 4., a balloon obscures the view for a fraction of a second, then it's a new angle, Ben talking to young David. In the end, we see that time traveller David is downstairs during this.

So what's going on here? Is that stealth cut actually long one, and Ben and David's meeting taking place during that? Or are there _two_ time machines, one in the basement and one with Ben? (And of course a third one in David's backpack)

The movie is so sloppy and inconsistent regarding time travel, that I think it's the former, but it still bugs me.
What do you think?